In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date:                (i) part of common general knowledge; or        (ii) known to be relevant to an attempt to solve any problem with which this specification is concerned.        
An omni-directional microphone converts sound waves emanating from all directions into electrical signals to be passed to an output. A directional microphone system is typically constructed from two or more omni-directional microphones, in a configuration that attenuates sounds emanating from certain directions and enhances sounds emanating from other directions.
The directionality of a particular directional microphone system in the horizontal plane is represented graphically by a polar pattern, where the direction directly in front of the microphone is shown at 0°, and the direction directly behind the microphone is shown at 180°. The plot of a polar pattern represents gain as a function of the direction of sound arrival, the gain for any given direction represented by the distance from the centre of the polar coordinates.
Some of the more common polar patterns are illustrated in FIG. 1, which shows an omni-directional polar pattern 10 (with no nulls), a bi-directional polar pattern 12 (with nulls at 90° and 270°), a cardioid polar pattern 14 (with a null at 180°) and a super-cardioid polar pattern 16 (with nulls at approximately 135° and 225°)
Directional microphone systems have been employed in the past in hearing aids to improve the signal-to-noise ratio. It is assumed the sound that the listener wishes to hear emanates from a forward direction, ie the direction in front of the listener, and so the directional microphone system is designed to provide a maximum gain for sounds emanating from this direction whilst attempting to reduce the sounds emanating from other directions.
Conventionally, directional microphone systems are fixed, meaning that the output signal has a fixed polar pattern. Fixed directional microphones traditionally comprise two spaced omni-directional microphones, a delay element and a difference element, and are configured to provide a fixed directional signal by subtracting the delayed signal from the original signal.
Examples of fixed directional microphone systems that do not utilise a delay element are disclosed in U.S. Pat. No. 5,463,694 and U.S. Pat. No. 4,712,244. These directional systems instead use a particular combination of averaging, amplifying, summing, subtracting and integrating elements that operate on the signals from the microphones to construct the fixed directional signal pattern.
As the output from a fixed directional microphone system is a polar pattern with a stationary null, it can only maximally attenuate sounds emanating from a particular direction (although sounds from directions close to the null will receive some attenuation). In many practical situations this can represent a significant compromise on the performance of the system. If noise emanates from a direction different to that of the null, or from multiple directions (which would require a compromise null position), or if there is a moving noise source, a reduced signal-to-noise ratio will result.
More complex ‘adaptive’ directional microphone systems have been developed to overcome shortcomings in directional microphone systems. Such systems have the ability to construct varying polar patterns which are able to dynamically ‘steer’ a null to attenuate signals representing sounds emanating from different directions, or from moving sources.
Known adaptive directional microphone systems are in fact extensions of conventional fixed systems, and typically utilise a variable delay element to vary the polar patterns, and thus provide adaptive directional signals. The architecture of such an adaptive directional microphone system is illustrated in FIG. 2. Front 20 and rear 22 omni-directional microphones transduce sound waves into front 21 and rear 23 electrical signals.
When a sound wave arrives from the forward direction, it reaches the front microphone first, and hence the rear signal 23 is a delayed version of the front signal 21. Likewise, if the sound arrives from behind, the front signal 21 is a delayed version of the rear signal 23. If the sound arrives from the side, there is no delay between the two signals 21 and 23. In short, the delay between the two signals is dependent on the angle of arrival of the sound wave. A variable delay element 24, coupled to the rear microphone 22, is used to match the delay corresponding to the desired cancellation direction. This produces a delayed rear signal 25. This signal 25 is received by a difference element 26 also coupled to the front microphone 20, configured as shown to output the difference between signals 21 and 25 to produce the directional output signal 30. As will be understood by those skilled in the art, the adaptive nature of this system is provided by a feedback loop, the adaptive directional signal 30 feeding back to an optimising algorithm element 28, which in turn provides an optimised delay value 29 to the variable delay element 24 used in producing delayed rear signal 25. The system is therefore designed to iteratively converge to a desired solution, in accordance with the algorithm implemented by element 28.
Various examples of known adaptive directional microphone systems that use variable delay elements are described in U.S. Pat. No. 5,757,933, US-2001/0028720, US-2001/0028718, U.S. Pat. No. 6,539,096 and U.S. Pat. No. 6,339,647. The main disadvantages of these systems are the complexity involved in implementing the variable delay element, along with the possible instability introduced through the use of a feedback structure.
Adaptive directional microphone systems that do not employ variable delay elements are also known, and examples of such systems are described in WO-01/97558 and US-2003/0031328. Both systems utilise two fixed delay elements to generate a forward-facing and a backward-facing cardioid polar pattern, which respectively represent an ‘enhanced signal’ and an ‘enhanced noise’. The enhanced noise and enhanced signal are then combined to produce an adaptive directional signal. An optimisation algorithm is used to find the ideal combination of the two signals to give maximum noise rejection. A major disadvantage of these adaptive directional systems is again their reliance on delay elements, in this case multiple fixed delay elements. As discussed above, these elements can be very difficult to implement in hardware, or require a specially designed allpass filter, which significantly increases the processing requirements of the system, particularly when implemented using a digital signal processor.
Adaptive directional microphone systems have also been developed that, instead of being continuously variable, simply select an output from a range of signals that have been implemented. One of the simplest approaches is described in U.S. Pat. No. 6,327,370, and involves using a fixed directional signal and an omni-directional signal, with a selection between the signals based on prescribed criteria such as ambient noise level. The idea has been extended in the teaching of U.S. Pat. No. 6,522,756, which includes a greater number of directional signals for selection. Such ‘signal selection’ systems are quite simple and can perform well, however for adequate performance they require many signals to be generated simultaneously, greatly increasing the demands on hardware and processing power. In addition, the limited choice of beam types signifies a discontinuous response, such that a signal with an optimum polar pattern cannot always be found.
There remains a need to provide an improved, or at least an alternative, method and apparatus for producing adaptive directional signals.